After estimating the value for each party for visibility, tonality and agenda setting at a monthly level for the different news providers, the deviation from a reference point is calculated. I use two different reference points and compare them. Depending on the choice if the reference point, the interpretation of the bias metric also differs.
To ensure comparability between the different metrics, they are standardized to range from −1 to 1.
We use the data from the “Sonntagsumfrage” (Sunday survey) from infratest dimap. The institution regularly asks at least 1000 German citizens the question: “Which party would you choose if federal elections take place next Sunday?” The survey thus measures the current election tendencies and therefore reflects an intermediate state in the opinion-forming process of the electoral population.
In this case the average value of all other parties in the month and the medium serves as the reference value to calculate the bias.